Lignocellulosic biomass is widely recognized as a promising source of raw material for production of renewable fuels and chemicals. The primary obstacle impeding the more widespread production of energy from biomass feedstocks is the general absence of low-cost technology for overcoming the recalcitrance of these materials to conversion into useful fuels. Lignocellulosic biomass contains carbohydrate fractions (e.g., cellulose and hemicellulose) that can be converted into ethanol. In order to convert these fractions, the cellulose and hemicellulose must ultimately be converted or hydrolyzed into monosaccharides; it is the hydrolysis that has historically proven to be problematic.
Biologically mediated processes are promising for energy conversion, in particular for the conversion of lignocellulosic biomass into fuels. Biomass processing schemes involving enzymatic or microbial hydrolysis commonly involve four biologically mediated transformations: (1) the production of saccharolytic enzymes (cellulases and hemicellulases); (2) the hydrolysis of carbohydrate components present in pretreated biomass to sugars; (3) the fermentation of hexose sugars (e.g., glucose, mannose, and galactose); and (4) the fermentation of pentose sugars (e.g., xylose and arabinose). These four transformations occur in a single step in a process configuration called consolidated bioprocessing (CBP), which is distinguished from other less highly integrated configurations in that it does not involve a dedicated process step for cellulase and/or hemicellulase production.
CBP offers the potential for lower cost and higher efficiency than processes featuring dedicated cellulase production. The benefits result in part from avoided capital costs, substrate and other raw materials, and utilities associated with cellulase production. In addition, several factors support the realization of higher rates of hydrolysis, and hence reduced reactor volume and capital investment using CBP, including enzyme-microbe synergy and the use of thermophilic organisms and/or complexed cellulase systems. Moreover, cellulose-adherent cellulolytic microorganisms are likely to compete successfully for products of cellulose hydrolysis with non-adhered microbes, e.g., contaminants, which could increase the stability of industrial processes based on microbial cellulose utilization. Progress in developing CBP-enabling microorganisms is being made through two strategies: engineering naturally occurring cellulolytic microorganisms to improve product-related properties, such as yield and titer; and engineering non-cellulolytic organisms that exhibit high product yields and titers to express a heterologous cellulase and hemicellulase system enabling cellulose and hemicellulose utilization.
Three major types of enzymatic activities are required for native cellulose degradation: The first type are endoglucanases (1,4-β-D-glucan 4-glucanohydrolases; EC 3.2.1.4). Endoglucanases cut at random in the cellulose polysaccharide chain of amorphous cellulose, generating oligosaccharides of varying lengths and consequently new chain ends. The second type are exoglucanases, including cellodextrinases (1,4-β-D-glucan glucanohydrolases; EC 3.2.1.74) and cellobiohydrolases (1,4-β-D-glucan cellobiohydrolases; EC 3.2.1.91). Exoglucanases act in a processive manner on the reducing or non-reducing ends of cellulose polysaccharide chains, liberating either glucose (glucanohydrolases) or cellobiose (cellobiohydrolase) as major products. Exoglucanases can also act on microcrystalline cellulose, presumably peeling cellulose chains from the microcrystalline structure. The third type are β-glucosidases (β-glucoside glucohydrolases; EC 3.2.1.21). β-Glucosidases hydrolyze soluble cellodextrins and cellobiose to glucose units.
A variety of plant biomass resources are available as lignocellulosics for the production of biofuels, notably bioethanol. The major sources are (i) wood residues from paper mills, sawmills and furniture manufacturing, (ii) municipal solid wastes, (iii) agricultural residues and (iv) energy crops. Pre-conversion of particularly the cellulosic fraction in these biomass resources (using either physical, chemical or enzymatic processes) to fermentable sugars (glucose, cellobiose and cellodextrins) would enable their fermentation to bioethanol, provided the necessary fermentative micro-organism with the ability to utilize these sugars is used.
On a world-wide basis, 1.3×1010 metric tons (dry weight) of terrestrial plants are produced annually (Demain, A. L., et al., Microbiol. Mol. Biol. Rev. 69, 124-154 (2005)). Plant biomass consists of about 40-55% cellulose, 25-50% hemicellulose and 10-40% lignin, depending whether the source is hardwood, softwood, or grasses (Sun, Y. and Cheng, J., Bioresource Technol. 83, 1-11 (2002)). The major polysaccharide present is water-insoluble, cellulose that contains the major fraction of fermentable sugars (glucose, cellobiose or cellodextrins).
Bakers' yeast (Saccharomyces cerevisiae) remains the preferred micro-organism for the production of ethanol (Hahn-Hagerdal, B., et al., Adv. Biochem. Eng. Biotechnol. 73, 53-84 (2001)). Attributes in favor of this microbe are (i) high productivity at close to theoretical yields (0.51 g ethanol produced/g glucose used), (ii) high osmo- and ethanol tolerance, (iii) natural robustness in industrial processes, (iv) being generally regarded as safe (GRAS) due to its long association with wine and bread making, and beer brewing. Furthermore, S. cerevisiae exhibits tolerance to inhibitors commonly found in hydrolyzates resulting from biomass pretreatment. The major shortcoming of S. cerevisiae is its inability to utilize complex polysaccharides such as cellulose, or its break-down products, such as cellobiose and cellodextrins.
Genes encoding cellobiohydrolases in T. reesei (cbh1 and cbh2), A. niger (cbhA and cbhB) and P. chrysosporium (cbh1-4) have been cloned and described. The proteins encoded by these genes are all modular enzymes containing a catalytic domain linked via a flexible liner sequence to a cellulose-binding module. Cbh1, Cbh2, CbhB and Cbh1-4 are family 7 glycosyl hydrolases. Glycosyl hydrolases are a widespread group of enzymes that hydrolyse the glycosidic bond between two or more carbohydrates, or between a carbohydrate and a non-carbohydrate moiety. A classification system for glycosyl hydrolases, based on sequence similarity, has led to the definition of 85 different families (Henrissat, B. et al., Proc. Natl. Acad. Sci. 92:7090-7094 (1995); Davies, G. and Henrissat, B., Structure 3: 853-859 (1995)). Glycoside hydrolase family 7 (GHF7) comprises enzymes with several known activities including endoglucanase (EC:3.2.1.4) and cellobiohydrolase (EC:3.2.1.91). These enzymes were formerly known as cellulase family C. Glycosyl hydrolase family 7 enzymes have a 67% homology at the amino acid level, but the homology between any of these enzymes and the glycosyl hydrolase family 6 CBH2 is less than 15%.
Exoglucanases and cellobiohydrolases play a role in the conversion of cellulose to glucose by cutting the dissaccharide cellobiose from the nonreducing end of the cellulose polymer chain. Structurally, cellulases and xylanases generally consist of a catalytic domain joined to a cellulose-binding module (CBM) via a linker region that is rich in proline and/or hydroxy-amino acids. In some cases, however, cellulases do not contain a CBM, and only contain a catalytic domain. Examples of such CBM-lacking cellulases include Cbhs from Humcola grisea, Phanerochaete chrysosporium and Aspergillus niger. Grassick et al., Eur. J. Biochem. 271: 4495-4506 (2004). In type I exoglucanases, the CBM domain is found at the C-terminal extremity of these enzyme (this short domain forms a hairpin loop structure stabilised by 2 disulphide bridges).
Classically, exoglucanases such as the cellobiohydrolases (Cbh) possess tunnel-like active sites, which can only accept a substrate chain via its terminal regions. These exo-acting Cbh enzymes act by threading the cellulose chain through the tunnel, where successive cellobiose units are removed in a sequential manner. Sequential hydrolysis of a cellulose chain is termed ‘processivity.’
Two of the better characterized Cbh members of GH7 are Cel7A from T. reesei and Cel7D (Cbh58) from P. chrysosporium. Both Cbhs consist of two β-sheets that pack face-to-face to form a β-sandwich. Cel7A from T. reesei is composed of long loops, one face of the sandwich that form a cellulose-binding tunnel. The catalytic residues are glutamate 212 and 217, which are located on opposite sides of the active site.
Several genes from the GH7 family of enzymes have been cloned and characterized from a variety of fungal sources, including H. grisea, T. reesei, T. aurantiacus, Penicillium janthinellum, P. chrysospirum and Aspergillus species. In addition, Cbh enzymes from T. emersonii, including Cbh1, have been isolated and characterized. The T. emersonii Cbh1 contains a secretory signal peptide and a catalytic domain. The CBM and linker region that are characteristic of some other GH family members are not present in the molecule.
With the aid of recombinant DNA technology, several of these heterologous cellulases from bacterial and fungal sources have been transferred to S. cerevisiae, enabling the degradation of cellulosic derivatives (Van Rensburg, P., et al., Yeast 14, 67-76 (1998)), or growth on cellobiose (Van Rooyen, R., et al., J. Biotech. 120, 284-295 (2005)); McBride, J. E., et al., Enzyme Microb. Techol. 37, 93-101 (2005)).
Related work was described by Fujita, Y., et al., (Appl. Environ. Microbiol. 70, 1207-1212 (2004)) where cellulases immobilised on the yeast cell surface had significant limitations. Firstly, Fujita et al. were unable to achieve fermentation of amorphous cellulose using yeast expressing only recombinant Bgl1 and EgII. A second limitation of the Fujita et al. approach was that cells had to be pre-grown to high cell density on standard carbon sources before the cells were useful for ethanol production using amorphous cellulose (e.g., Fujita et al. teaches high biomass loadings of ˜15 g/L to accomplish ethanol production).
As noted above, ethanol producing yeast such as S. cerevisiae require addition of external cellulases when cultivated on cellulosic substrates, such as pre-treated wood, because this yeast does not produce endogenous cellulases. Expression of fungal cellulases such as T. reesei Cbh1 and Cbh2 in yeast S. cerevisiae have been shown to be functional. Den Haan, R., et al., “Functional expression of cellobiohydrolases in Saccharomyces cerevisiae towards one-step conversion of cellulose to ethanol,” Enzyme and Microbial Technology 40:1291-1299 (2007). However current levels of expression and specific activity of cellulases heterologously expressed in yeast are still not sufficient to enable growth and ethanol production by yeast on cellulosic substrates without externally added enzymes. While studies have shown that perhaps certain cellulases, such as T. reesei Cbh1 have specific activity when heterologously expressed, there remains a significant need for improvement in the amount of Cbh activity expressed in order to attain the goal of achieving a consolidated bioprocessing (CBP) system capable of efficiently and cost-effectively converting cellulosic substrates to ethanol.
In order to address the limitations of heterologous Cbh expression in consolidated bioprocessing systems, the present invention provides for heterologous expression of wild-type and codon-optimized variants of Cbh1 and/or Cbh2 from the fungal organisms Talaromyces emersonii (T. emersonii), Humicola grisea (H. grisea), Thermoascus aurantiacus (T. aurantiacus), and Trichoderma reesei (T. reesei) in host cells, such as the yeast Saccharomyces cerevisiae. The expression in such host cells of the corresponding genes, and variants and combinations thereof, result in improved specific activity of the expressed cellobiohydrolases. Thus, such genes and expression systems are useful for efficient and cost-effective consolidated bioprocessing systems.